shouldnt be too hard to build a entry level system for £400
firstly lets go through useable parts..
what memory do you currently use (DDR2 can be ported over to a new board, DDR3 from what l remember is still a lil pricey.. tho l havent looked recently)
if you recently replaced your power supply, how powerful is it?
CD/DVD drive, if its still going strong shouldnt need to replace it, same with hard drives unless you want a new one to install to so you can save yourself backing up data on your current OS drive (that being said are they SATA or PATA aka IDE? SATA is allot easier to work with these days as boards seem to be built more for SATA these days)
ideally tho depending on your hardware, if its not totally outdated you maybe able to salvage parts meaning the likely replacements would only be CPU, motherboard, and GFX which l can see costing under £200 depending on how far you want to take your system like quad core processors and more mainstream gamer GFX cards, equally ld expect if you went intel processors may cost a little more than ld be willing to pay as lm basing my estimations on AMD chips (as lve always been an ADM fan, never owned a system with an intel in it)
regardless, please list the possibly usable hardware such as your current ram, power supply, any drives (and their connectability SATA/PATA?) working from what is usable will provide a good estimate as to how far you can take your upgrade since you have a general price range, maybe you can expand upon what you have with something a little faster than you originally would have chosen on a complete upgrade (still l prolly dont have to say it but even your case should be usable)
as for the ebay links, yeah you can get some interesting stuff there.. but ld much rather have stuff from a place like ebuyer.com, your less likely to be ripped off and end up with broken stuff after transit..
still food for thought, considering what you currently have as possibilities because if some hardware can be used, you can expand upon what you currently have with something better to compliment it (kinda like l did, tho part of the reason was motherboard failure l pretty much jumped from a 1ghz dual core to a 3ghz dual core, and since ram was usable and l only needed a board, l got everything done for under £200 including new PSU, tho lm not counting my new case in that estimate tho it was high-street prices in this case)